Carbonated beverages such as sodas (also known as soft drinks) contain dissolved carbon dioxide. Carbonated beverages are commonly dispensed in restaurants, cafeterias, and other locations in dispensing machines that mix a concentrated syrup with water and carbon dioxide gas. The carbon dioxide is typically stored in pressurized containers in which it exists as both a liquid and a gas.
The behavior of the carbon dioxide in these pressurized containers follows well known physical laws. In a sealed system containing both a pure liquid and gas, the pressure is determined by the fluid's vapor pressure which, in turn, is a function of the temperature. In other words, the pressure of an isothermal (constant-temperature) sealed system is constant as long as both liquid and gas are present in the system. If so little fluid is present that there is no liquid at equilibrium, the pressure in the system will be less than the vapor pressure. If so much fluid is present that there is no gas at equilibrium, the pressure in the system may greatly exceed the vapor pressure.
Although the vapor pressure remains constant for a constant temperature in a sealed system containing both liquid and gas, the relative amounts of liquid and gas present at equilibrium are a function of the amount of fluid in the sealed system. For example, if the system contains enough fluid to have only a small amount liquid present, almost all the volume of the system will be occupied by gas. Similarly, if the system is nearly filled with liquid, only a small volume of gas will be present.
The vapor pressure of carbon dioxide in pounds force per square inch absolute (psia) is shown in the following table.
TABLE 1Vapor Pressure of Carbon DioxideTemperature (° C.)Temperature (° F.)Pressure (psia)−155330−1014380−523440032500541580105065015597402068830257793030861050
The “critical temperature” of carbon dioxide is 31° C. (88° F.) at which point its vapor pressure is about 1070 psia. If the critical temperature is exceeded, liquid and gas no longer exist in equilibrium and the pressure in the system may greatly exceed 1070 psia.
The physical properties of sealed systems of carbon dioxide liquid and gas influence the way in which carbon dioxide gas is provided to beverage dispensers. A conventional carbon dioxide system is disclosed in Sloan et al., U.S. Pat. No. 6,817,385, Nov. 16, 2004, which is incorporated by reference. The refilling of the Sloan et al. system is illustrated in FIG. 1. A supply of refrigerated liquid carbon dioxide is brought to the location in a truck or other pump supply 16. The temperature of the carbon dioxide in the truck is generally near 0° F. and the pressure is about 300 psia. A pump 14 is connected and a vent valve 18 in supply line 12 is positioned to establish communication between the pump supply and a carbon dioxide manifold 10. The carbon dioxide manifold 10 contains an internal valve that is closed during refilling so that the liquid carbon dioxide flows out of the manifold through line 48 and into two liquid containers 50 equipped with valves 52. A line 42 leads to a relief valve 44. Liquid carbon dioxide is added until the liquid containers are full. The vent valve is then opened and the pump supply is removed.
At this point, the internal valve in the carbon dioxide manifold is supposed to open to stop flow from the manifold back through the supply line and to provide communication between the liquid containers and the gas container 70 via line 68 and valve 72. If and when the valve opens, liquid carbon dioxide changes to gas and fills the gas container until the pressure equals the vapor pressure for the temperature of the system. The ratio of two volumes of liquid carbon dioxide to one volume of gaseous carbon dioxide is mandated by governmental regulations and is commonly used. If the ratio is higher, the volume of gaseous carbon dioxide available for immediate use may not be sufficient. If the ratio is lower, the system must be refilled more frequently. The two-to-one ratio also provides a margin of safety to ensure that the system is not completely filled with liquid carbon dioxide. As discussed above, the pressure in the system may greatly exceed the vapor pressure if no gas is present.
The carbon dioxide gas in the gas container is withdrawn through line 76 to carbonate beverages at the beverage dispenser. The system remains at the vapor pressure for the given temperature until all the liquid carbon dioxide has changed to gas. After this point, any subsequent flow of carbon dioxide from the system will decrease the pressure in the system. The decrease in pressure is noted on pressure gauge 64 which is connected to the manifold by line 62 and the system is refilled before the system is empty.
FIGS. 2 and 3 illustrate the manifold in more detail. The manifold contains internal passages that communicate with the pump supply via line 12, with the liquid containers via line 48, with the relief valve via line 42, with the gas container via line 68, with the pressure gauge via line 62, and with the beverage dispenser via line 76. The manifold contains a sliding spool valve 32 that is designed to move between open/operating and closed/refilling positions in response to the relative pressures in the system. The spool valve contains an internal axial passage 78 that provides the communication between line 12 and the rest of the system. When the spool valve is the open/operating position shown in FIG. 2, communication between the liquid and gas containers takes place, and communication through the axial passage is blocked. When the spool valve is in the closed/refilling position shown in FIG. 3, communication between the liquid and gas containers is prevented, and communication through the axial passage is allowed.
As discussed above, the spool valve closes automatically during refilling because of the pressure from the pump supply. The spool valve is supposed to open after refilling. The force to open the spool valve is provided by the residual pressure in the gas container. Most of the time this pressure is greater than the pressure in the liquid containers (which is about 500 psia if the carbon dioxide warms to a temperature of about 32° F. during refilling) and is sufficient to open the valve. However, the residual pressure in the gas container may be less than the pressure in the liquid containers under certain circumstances.
As one example of excessively low pressure in the gas container(s), the pressure in the gas container is at atmospheric pressure (14.7 psia) when the system is first filled. As a second example, the beverage dispensing system may develop a leak and all the carbon dioxide may escape. As a third example, the carbon dioxide may be withdrawn by persons operating the beverage dispensing system until the system is nearly empty. Regardless of how it happens, if the residual pressure in the gas container(s) is less than about 500 psia, the pressure differential is insufficient to open the spool valve. A second embodiment of the Sloan et al. manifold contains a spring to provide an additional opening force. However, even with the additional force of the spring, the spool valve can remain closed after refilling if the residual pressure in the gas container is very low. If the spool valve does not open, carbon dioxide will not flow from the liquid container(s) into the gas container. Instead, it will flow out of the system through supply line 12 and vent valve 18.
Accordingly, there is a demand for a carbon dioxide manifold that reliably opens after refilling even if the residual pressure in the gas container(s) is very low.